<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    body>div{
      width: 200px; height: 200px; border: 1px solid red;
      /*当元素的所有子集全部浮动时自动识别的高度为0，此时后面的元素会顶上来导致元素层叠的显示异常，
      给元素添加overflow: hidden;解决问题*/
        overflow: hidden;
    }
    div>div{
      width: 50px; height: 50px;
    }
    #d1{
      background-color: red;
      float: left;
    }
    #d2{
      background-color: green;
      float: left;
    }
    #d3{
      background-color: blue;
      float: left;
    }
    ul{
        list-style-type: none;
      margin: 0;
      padding: 0;
        overflow: hidden;
    }
    li{
        float: left;
        margin-right: 20px;
    }

  </style>
</head>
<body>
<ul>
 <li><a href="">首页</a></li>
 <li><a href="">免费课</a></li>
 <li><a href="">直播课</a></li>
 <li><a href="">精品课</a></li>
 <li><a href="">线上课</a></li>
</ul>
<div>
  <div id="d1"></div>
  <div id="d2"></div>
  <div id="d3"></div>
</div>

</body>
</html>